Recently, as a new material system that would greatly expand the utilization field of III–V compound semiconductors, there has been proposed a III–V compound semiconductor material comprising, as the V-group composition, nitrogen (N) at a mole fraction of not more than several % of the V-group content, and arsenic (As) and phosphorus (P). V-group elements such as As and P greatly differ from N in the atomic radius (N: 0.070 μm, As: 0.118 nm, P: 0.110 nm), and in the elcotronegativity (N: 3.5, As: 2.4, P: 2.5). Thus, by intermixing N and As, or N and P, or N and As and P, a singular physical property is generated. For example, in the case of GaInNAs, which is presumed to be an intermixture of GaInAs and several percent of GaInN having a larger bandgap width than that of GaInAs, bowing in the change of the bandgap width due to the intexmixing is extremely large. Thus, there is observed a phenomenon in which, although several percent of a system having a large bandgap width is intermixed, the bandgap width is nevertheless rapidly narrowed in conjunction with the intermixing.
GaInNAs thus obtained is industrially important because it can be used for a light-emitting layer of a light-emitting device that emits light at a wavelength of 1.3 μm, 1.55 μm or a longer wavelength, which are important wavelengths for optical fiber communication, while generally lattice-matched with an inexpensive and good GaAs substrate (or with a strain amount of not more than several percent).
Further, Japanese Journal of Applied Physics, Vol. 35, part 1, No. 2B, February 1996, pp 1273–1275 (first prior art) discloses that, in the case where a III–V compound semiconductor material in which N and As, or N and P, or N and As and P are intermixed, such as GaInNAs, is used as a material of an active layer of a semiconductor laser, the temperature characteristic of the semiconductor laser is remarkably improved. That is, in the case where part of an As content in GaInAs is replaced with an N content, the conduction band energy level in the bandgap of this mixed crystal semiconductor material is lowered, a conduction band energy difference ΔEc in a hetero junction between the material and other materials such as GaAs increases. Therefore, confinement of electrons in the mixed crystal semiconductor material used as an active layer is remarkably enhanced, so that the characteristic temperature To of the semiconductor laser is remarkably increased.
In Photonics Technology Letters, Vol. 10, No. 4, April 1998, p. 487 (second prior art), a semiconductor laser having the above construction is shown more specifically. That is, it has been reported that a semiconductor laser structure, wherein an active layer consisting of Ga0.7In0.3N0.01As0.99 quantum well layers and GaAs guide layers is interposed between Al0.3Ga0.7As upper and lower cladding layers, was fabricated on a GaAs substrate, and that the semiconductor laser achieved a 1.31 μm continuous-wave lasing operation at room temperature for the first time among semiconductor lasers constructed of a system lattice-matching with the GaAs substrate.
In the mixed crystal system containing, as V-group contents, N and other V-group elements (such as As and P), there is a very large non-miscible region (miscibility gap), which does not form a stable mixed crystal system in a thermal equilibrium state. Thus, it becomes difficult to mix nitrogen in a crystal. For that reason, as the N mole fraction in the mixed crystal increases, a problem of rapid deterioration of light-emitting characteristics occurs. According to the study of the inventor of the present application, it has been found that, as the N mole fraction increases, light intensity of photoluminescence exponentially deteriorates.